1628 XXXI International Mineral Processing Congress 2024 Proceedings/Washington, DC/Sep 29–Oct 3
Bryson and Crundwell, 2014 Crundwell et al., 2015).
Some of the results and arguments for this model will be
discussed in the next section. Prior to doing this, some criti-
cisms will be discussed.
The first criticism of this semiconductor model, and
the one most frequently raised, is that the semiconductor
effects are only applicable to highly purified substances. A
sulphide mineral in contact with a metal wire was used as a
rectifying diode (a property of semiconductors) to receive
radio signals without the need for external power. The min-
eral used was galena, but pyrite and chalcopyrite were also
used. Pyrite was found to be easier to adjust and suited to
the signal strength. Such crystal sets were sold in their mil-
lions during the 1920s before they were displaced by sets
with amplifiers.
A second criticism of this model was raised when
Crundwell et al. (2015) responded to this first criticism by
showing that chalcopyrite electrodes responded to light,
another property of semiconductors. Nicol (2016) claimed
that the results that they presented where not a feature of
the semiconducting nature of chalcopyrite, but a result of
heating. However, an analysis of the results indicated that
the activation energy for dissolution would need to be
about 200 kJ/mol if Nicol was correct. However, the acti-
vation energy is less than half of this value, making Nicol’s
argument somewhat spurious. In addition, the results
of Crundwell et al. (2015) are dependent on voltage, in
line with the photocurrent effect, which does not support
Nicol’s criticism that their results were a heating effect since
a thermal effect would not be dependent on voltage.
A third criticism was raised early in the development
of these ideas: that the conduction type (n- or p-type) does
not seem to affect the dissolution behavior. However, the
semiconductor model as formulated by Crundwell (1988
a, b) is not predicated on the conduction type. Instead, it is
based on an incorporation of the electronic structure of the
mineral into developing a more complete understanding of
dissolution of these minerals. In addition, Crundwell and
Bryson (2014) have shown how the dissolution behavior
is influenced by surface states, a feature of semiconductor
surfaces, so that some results are not affected by conduction
type.
A fourth criticism was raised by Nicol (2016) in which
he observed strong absorption of light on a chalcopyrite
electrode, particularly violet light. He argued that this was
evidence of the semiconducting nature of the passive layer.
However, chalcopyrite has a brassy-yellow colour, indicat-
ing that it strongly absorbs light in the blue and violet end.
Nicol’s observation supports the semiconductor model, in
direct contrast to his interpretation.
In the next section, the application of the semiconduc-
tor model of dissolution to sphalerite, pyrite and chalcopy-
rite is discussed.
APPLICATION OF THE
SEMICONDUCTOR MODEL
TO SPHALERITE, PYRITE AND
CHALCOPYRITE
Introduction
The immersion of the solid in the electrolyte during leach-
ing causes the charge at the surface to redistribute across
the interface with an equal density of charge in the solid,
called the space-charge layer, and the solution (called the
Gouy layer). If changes in either potential or electrolyte
conditions do not cause a change in the charge of the
space-charge layer, the Fermi level (the thermodynamic
work required to add an additional electron to the solid) is
said to be pinned. Because electron transfer can only occur
between states of the same energy level, a comparison of the
energy levels gives a visual picture of the thermodynamics
of the dissolution (see Figure 3). However, since kinetics are
more important than thermodynamics in the field of dis-
solution, a more detailed understanding is required.
Sphalerite, Its Color and Its Rate of Dissolution
Sphalerite has an impurity of iron in it, which substitutes
for zinc in the structure (Zn,Fe)S. The concentration of
iron in substitutional sites affects the colour of the min-
eral. At low concentrations of iron, sphalerite is known as
cleiophane, while at concentrations above 10% it is known
as marmatite, and at concentration up to 26% is might be
called christophite. The name sphalerite comes from the
Greek for ‘deceiving’ and the early German name ‘blende’
also meant to ‘blind’ or ‘deceive’.
The presence of iron in substitutional positions for zinc
results in two additional bands within the band gap. These
2e and 4t2 bands arise from the d-orbitals of iron at about
0.56 and 1.44 eV above the valence band. These iron-impu-
rity bands absorb light in the violet-blue parts of the visible
spectrum, so that low iron samples appear honey-coloured
or orange. These colour transitions are shown in Figure 4.
The iron impurity is significant, because it does not
only change the colour, it also changes the rate of disso-
lution. Piao and Tozawa (1985) and Crundwell (1988a)
showed that the rate of dissolution is directly propor-
tional to the iron content. These results are also shown in
Figure 5a. Crundwell (1988a, b) derived a model based on
quantum electrochemistry that describes the effect of the
iron impurity, and the concentration of oxidant in solution.
Bryson and Crundwell, 2014 Crundwell et al., 2015).
Some of the results and arguments for this model will be
discussed in the next section. Prior to doing this, some criti-
cisms will be discussed.
The first criticism of this semiconductor model, and
the one most frequently raised, is that the semiconductor
effects are only applicable to highly purified substances. A
sulphide mineral in contact with a metal wire was used as a
rectifying diode (a property of semiconductors) to receive
radio signals without the need for external power. The min-
eral used was galena, but pyrite and chalcopyrite were also
used. Pyrite was found to be easier to adjust and suited to
the signal strength. Such crystal sets were sold in their mil-
lions during the 1920s before they were displaced by sets
with amplifiers.
A second criticism of this model was raised when
Crundwell et al. (2015) responded to this first criticism by
showing that chalcopyrite electrodes responded to light,
another property of semiconductors. Nicol (2016) claimed
that the results that they presented where not a feature of
the semiconducting nature of chalcopyrite, but a result of
heating. However, an analysis of the results indicated that
the activation energy for dissolution would need to be
about 200 kJ/mol if Nicol was correct. However, the acti-
vation energy is less than half of this value, making Nicol’s
argument somewhat spurious. In addition, the results
of Crundwell et al. (2015) are dependent on voltage, in
line with the photocurrent effect, which does not support
Nicol’s criticism that their results were a heating effect since
a thermal effect would not be dependent on voltage.
A third criticism was raised early in the development
of these ideas: that the conduction type (n- or p-type) does
not seem to affect the dissolution behavior. However, the
semiconductor model as formulated by Crundwell (1988
a, b) is not predicated on the conduction type. Instead, it is
based on an incorporation of the electronic structure of the
mineral into developing a more complete understanding of
dissolution of these minerals. In addition, Crundwell and
Bryson (2014) have shown how the dissolution behavior
is influenced by surface states, a feature of semiconductor
surfaces, so that some results are not affected by conduction
type.
A fourth criticism was raised by Nicol (2016) in which
he observed strong absorption of light on a chalcopyrite
electrode, particularly violet light. He argued that this was
evidence of the semiconducting nature of the passive layer.
However, chalcopyrite has a brassy-yellow colour, indicat-
ing that it strongly absorbs light in the blue and violet end.
Nicol’s observation supports the semiconductor model, in
direct contrast to his interpretation.
In the next section, the application of the semiconduc-
tor model of dissolution to sphalerite, pyrite and chalcopy-
rite is discussed.
APPLICATION OF THE
SEMICONDUCTOR MODEL
TO SPHALERITE, PYRITE AND
CHALCOPYRITE
Introduction
The immersion of the solid in the electrolyte during leach-
ing causes the charge at the surface to redistribute across
the interface with an equal density of charge in the solid,
called the space-charge layer, and the solution (called the
Gouy layer). If changes in either potential or electrolyte
conditions do not cause a change in the charge of the
space-charge layer, the Fermi level (the thermodynamic
work required to add an additional electron to the solid) is
said to be pinned. Because electron transfer can only occur
between states of the same energy level, a comparison of the
energy levels gives a visual picture of the thermodynamics
of the dissolution (see Figure 3). However, since kinetics are
more important than thermodynamics in the field of dis-
solution, a more detailed understanding is required.
Sphalerite, Its Color and Its Rate of Dissolution
Sphalerite has an impurity of iron in it, which substitutes
for zinc in the structure (Zn,Fe)S. The concentration of
iron in substitutional sites affects the colour of the min-
eral. At low concentrations of iron, sphalerite is known as
cleiophane, while at concentrations above 10% it is known
as marmatite, and at concentration up to 26% is might be
called christophite. The name sphalerite comes from the
Greek for ‘deceiving’ and the early German name ‘blende’
also meant to ‘blind’ or ‘deceive’.
The presence of iron in substitutional positions for zinc
results in two additional bands within the band gap. These
2e and 4t2 bands arise from the d-orbitals of iron at about
0.56 and 1.44 eV above the valence band. These iron-impu-
rity bands absorb light in the violet-blue parts of the visible
spectrum, so that low iron samples appear honey-coloured
or orange. These colour transitions are shown in Figure 4.
The iron impurity is significant, because it does not
only change the colour, it also changes the rate of disso-
lution. Piao and Tozawa (1985) and Crundwell (1988a)
showed that the rate of dissolution is directly propor-
tional to the iron content. These results are also shown in
Figure 5a. Crundwell (1988a, b) derived a model based on
quantum electrochemistry that describes the effect of the
iron impurity, and the concentration of oxidant in solution.